In the Gregorian calendar, years that are divisible by 100, but not by 400, do not contain a leap day. Thus, 1700, 1800, and 1900 did not contain a leap day; neither will 2100, 2200, and 2300. Conversely, 1600 and 2000 did and 2400 will. Years containing a leap day are called leap years. February 29 is the 60th day of the Gregorian calendar in such a year, with 306 days remaining until the end of the year. In the Chinese calendar, this day will only occur in years of the monkey, dragon, and rat.

A leap day is observed because a complete revolution around the Sun takes approximately 6 hours longer than 365 days (8,760 hours). A leap day compensates for this lag, realigning the calendar with the Earth's position in the Solar System; otherwise, seasons would occur earlier than intended in the calendar year. Originally, the Julian calendar added a leap day every four years, but this turned out to add too many days, making the equinoxes and solstices shift gradually to earlier dates. As the shifting became noticeable (by the late 16th century the vernal equinox had drifted to March 11) the Gregorian calendar was introduced both to shift it back by omitting several days, and to reduce the number of leap years via the "century rule" to keep the equinoxes more or less fixed and the date of Easter consistently close to the vernal equinox.[1][2]

Although most modern calendar years have 365 days, a complete revolution around the Sun (one solar year) takes approximately 365 days and 6 hours. An extra 24 hours thus accumulates every four years, requiring that an extra calendar day be added to align the calendar with the Sun's apparent position. Without the added day, in future years the seasons would occur later in the calendar, eventually leading to confusion about when to undertake activities dependent on weather, ecology, or hours of daylight.

A solar year is actually slightly shorter than 365 days and 6 hours (365.25 days). As early as the 13th century it was recognized that the year is shorter than the 365.25 days assumed by the Julian calendar: the Earth's orbital period around the Sun was derived from the medieval Alfonsine tables as 365 days, 5 hours, 49 minutes, and 16 seconds (365.2425 days). The currently accepted modern figure is 365 days, 5 hours, 48 minutes, 45 seconds. Adding a calendar day every four years, therefore, results in an excess of around 44 minutes for those four years, or about 3 days every 400 years. To compensate for this, three days are removed every 400 years. The Gregorian calendar reform implements this adjustment by making an exception to the general rule that there is a leap year every four years. Instead, a year divisible by 100 is not a leap year unless that year was also exactly divisible by 400. This means that the years 1600, 2000, and 2400 are leap years, while the years 1700, 1800, 1900, 2100, 2200, 2300, and 2500 are common years.

The Gregorian calendar repeats itself every 400 years, which is exactly 20,871 weeks including 97 leap days. Over this period, February 29 falls on Sunday, Tuesday, and Thursday 13 times each; 14 times each on Friday and Saturday; and 15 times each on Monday and Wednesday. Excepting when a century mark that is not a multiple of 400 intervenes, consecutive leaps days fall in order Thursday, Tuesday, Sunday, Friday, Wednesday, Monday, and Saturday; then repeating with Thursday again.

Adding a leap day (after 23 February) shifts the commemorations in the 1962 Roman Missal.

The calendar of the Roman king Numa Pompilius had only 355 days (even though it was not a lunar calendar) which meant that it would quickly become unsynchronized with the solar year. An earlier Roman solution to this problem was to lengthen the calendar periodically by adding extra days to February, the last month of the year. February consisted of two parts, each with an odd number of days. The first part ended with the Terminalia on the 23rd, which was considered the end of the religious year, and the five remaining days formed the second part. To keep the calendar year roughly aligned with the solar year, a leap month, called Mensis Intercalaris ("intercalary month"), was added from time to time between these two parts of February. The (usual) second part of February was incorporated in the intercalary month as its last five days, with no change either in their dates or the festivals observed on them. This followed naturally, because the days after the Ides (13th) of February (in an ordinary year) or the Ides of Intercalaris (in an intercalary year) both counted down to the Kalends of March (i.e. they were known as "the nth day before the Kalends of March"). The Nones (5th) and Ides of Intercalaris occupied their normal positions.

The third-century writer Censorinus says:

When it was thought necessary to add (every two years) an intercalary month of 22 or 23 days, so that the civil year should correspond to the natural (solar) year, this intercalation was in preference made in February, between Terminalia [23rd] and Regifugium [24th].[3]

The leap day was introduced in Rome as a part of the Julian reform in the 1st century BC. As before, the intercalation was made after February 23. The day following the Terminalia (February 23) was doubled, forming the "bis sextum"—literally 'twice sixth', since February 24 was 'the sixth day before the Kalends of March' using Roman inclusive counting (March 1 was the Kalends of March and was also the first day of the calendar year). Although there were exceptions, the first day of the bis sextum (February 24) was usually regarded as the intercalated or "bissextile" day since the 3rd century AD.[4] February 29 came to be regarded as the leap day when the Roman system of numbering days was replaced by sequential numbering in the late Middle Ages.

A person born on February 29 may be called a "leapling", a "leaper", or a "leap-year baby".[5] In non-leap years, some leaplings celebrate their birthday on either February 28 or March 1, while others only observe birthdays on the authentic intercalary date, February 29.

The effective legal date of a leapling's birthday in non-leap years varies between jurisdictions.

In the United Kingdom and Hong Kong, when a person born on February 29 turns 18, they are considered to have their birthday on March 1 in the relevant year.[6][7]

In New Zealand, a person born on February 29 is deemed to have their birthday on February 28 in non-leap years, for the purposes of Driver Licensing under §2(2) of the Land Transport (Driver Licensing) Rule 1999.[8] The net result is that for drivers aged 75, or over 80, their driver licence expires at the end of the last day of February, even though their birthday would otherwise fall on the first day in March in non-leap years. Otherwise, New Zealand legislation is silent on when a person born on 29 February has their birthday, although case law[9] would suggest that age is computed based on the number of years elapsed, from the day after the date of birth, and that the person's birth day then occurs on the last day of the year period. This differs from English common law where a birthday is considered to be the start of the next year, the preceding year ending at midnight on the day preceding the birthday. While a person attains the same age on the same day, it also means that, in New Zealand, if something must be done by the time a person attains a certain age, that thing can be done on the birthday that they attain that age and still be lawful.

If a period fixed by weeks, months, and years does not commence from the beginning of a week, month, or year, it ends with the ending of the day which proceeds the day of the last week, month, or year which corresponds to that on which it began to commence. But if there is no corresponding day in the last month, the period ends with the ending of the last day of the last month.[10]

Thus, in England and Wales or in Hong Kong, a person born on February 29 will have legally reached 18 years old on March 1. If he or she was born in Taiwan he or she legally becomes 18 on February 28, a day earlier. In the United States, according to John Reitz, a professor of law at the University of Iowa, there is no "... statute or general rule that has anything to do with leap day."[11] Reitz speculates that "March 1 would likely be considered the legal birthday in non-leap years of someone born on leap day,"[11] using the same reasoning as described for the United Kingdom and Hong Kong.

There are many instances in children's literature where a person's claim to be only a quarter of their actual age turns out to be based on counting their leap-year birthdays.

A similar device is used in the plot of Gilbert and Sullivan's 1879 comic operaThe Pirates of Penzance. As a child, Frederic was apprenticed to a band of pirates until his 21st birthday. Having passed his 21st year, he leaves the pirate band and falls in love. However, since he was born on February 29, his 21st birthday will not arrive until he is eighty-four, so he must leave his fiancée and return to the pirates.[12]

This plot point was also used in a Sherlock Holmes story based on the Basil Rathbone era, where a friend of Dr. Watson's is a Baronet who is due to receive his inheritance on the New Year's Day of the year where his twenty-first birthday will be celebrated, only for the law to deprive him of the money as he was born on February 29; with the 84-year-old Baronet distraught at the news that 1900 is not a leap year, Holmes helps the Baronet fake his death long enough for his grandson — who is the appropriate age to receive the inheritance — to establish his claim and receive the money himself.[citation needed]

There is a popular tradition known as Bachelor's Day in some countries allowing a woman to propose marriage to a man on February 29.[13] If the man refuses, he then is obliged to give the woman money[14] or buy her a dress. In upper-class societies in Europe, if the man refuses marriage, he then must purchase 12 pairs of gloves for the woman, suggesting that the gloves are to hide the woman's embarrassment of not having an engagement ring. In Ireland, the tradition is supposed to originate from a deal that Saint Bridget struck with Saint Patrick.[15][16]

^"A Convenient Year for a Leap". Wisconsin State Journal (Madison, Wisconsin). February 28, 1992. "Tradition dies hard in Aurora , Ill., where every Feb. 29 single women are deputized and allowed to arrest bachelors and fine them $4."

^Krucoff, Carol. "By Leaps and Seconds: It's Feb. 29: Call It a Time-Consuming Day", The Washington Post. February 29, 1984, p. F9. "Leap Day was not created, as rumor has it, to give women one day out of 1,461 to chase men (who needs a special day for that?)—even though the town of Aurora, Ill., deputizes single women and allows them to arrest bachelors (fine, $4) every Feb. 29."